1. Field of the Invention
The present invention relates in general to sigma-delta modulators and, more particularly, to a method and to a circuit for detecting an instability condition of a sigma-delta modulator of an order greater than one and for re-establishing stable operating conditions of the modulator.
2. Description of the Related Art
As is known, sigma-delta modulation is a technique which can achieve a high-resolution analog-digital conversion. According to this technique, an analog signal is xe2x80x9cover-sampledxe2x80x9d, that is, it is sampled at a rate much greater than that which is necessary for a conventional analog-digital converter operating at the Nyquist frequency. A sigma-delta modulator integrates the analog signal and performs a conventional delta modulation on the integral of the signal. For this purpose, the modulator uses a low-resolution quantizer. The output signal of the quantizer is added, with its sign inverted, to the analog input signal, by a feedback loop containing a digital-analog converter.
In order to produce a digital signal corresponding to the analog input signal, the signal output by the modulator is subjected to the action of a so-called digital decimation filter which produces a digital output signal at a sampling frequency equal to twice the Nyquist frequency. By virtue of the over-sampling and of the digital decimation, this technique achieves greater resolution and less distortion than can be achieved by the Nyquist-frequency conversion technique. Moreover, since the error signal, that is, the difference between the input signal and the sampled signal, is integrated, the sigma-delta modulator acts as a low-pass filter for the signal and as a high-pass filter for the quantization noise. It therefore attenuates the noise in the signal band particularly effectively.
The noise attenuation is appreciable even with a first-order sigma-delta modulator, that is, a modulator comprising a single integrator upstream of the quantizer, but to achieve the high signal/noise ratios required of high-resolution analog-digital converters, it is necessary to use higher-order modulators, that is, modulators comprising several integrators in cascade.
However, if a higher-order modulator is used, stability problems arise. Stability is determined by the type of input signal, in particular, by signals which exceed the input range of the circuit, by initial conditions upon switching-on, and by the possible presence of fluctuations in the supply voltage.
As soon as a departure from ideal operative conditions takes place and, more precisely, when the gain of an element of the feedback loop falls below a certain limit, the modulator becomes unstable and tends to oscillate. The quantizer is an element of the feedback loop, the gain of which is subject to variations with variations in the operating conditions. Conditions are referred to as unstable when the voltages of the internal analog nodes reach values higher than the maximum design range. In order to re-establish conditions of stability, intervention from outside the circuit is required.
For low-pass sigma-delta modulators, various methods have been proposed and differ from one another in the way in which instability is detected and in the action undertaken to re-establish stability.
A first method provides for the connection of limiter elements in parallel with the capacitors of the integrators. The selection of the thresholds of the limiters is critical; in fact, if the thresholds are close to the limits of the dynamic range of the operational amplifiers of the integrators, the signal may also be limited during normal operation with high input-signal levels, causing distortion; if, on the other hand, the thresholds are too low, there is a low signal/noise ratio. This solution cannot therefore be used in applications in which linearity is essential and a high value of the signal/noise ratio is required.
A second method provides for detection of the oscillation which occurs in instability conditions and for resetting of the state variables of the modulator as soon as the oscillation is detected.
In order to detect instability, it has been proposed to measure the analog voltages of the internal nodes of the circuit in order to compare them with respective predetermined reference values. If the reference values are exceeded, the system is considered unstable. A method of detecting instability by monitoring the digital output signal has also been proposed; more particularly, it has been proposed to define a sequence of bits which corresponds to an instability state and to monitor the output bit-stream in order to recognize the appearance of such a sequence and consequently to indicate an instability state.
In both methods, as soon as instability is detected, the output voltages of all of the integrators are reset so that, if the cause of the instability has ceased, the operation of the modulator is returned to stable operating conditions. This latter operation may require a relatively long recovery time, which is longer the higher is the order of the modulator and the larger is its input dynamic range.
The embodiments of the present invention overcome the limitations of the prior art with a method and a circuit for re-establishing the stability of a sigma-delta modulator of an order greater than one, within very short times.
This aim is achieved by the implementation of the method and by the provision of a circuit that includes a sigma-delta modulator having a plurality of integrator stages in cascade and a quantizer, and a logic control unit connected to the output of the quantizer and to respective resetting inputs of their integrator stages, the logic unit including a circuit for storing a bit sequence corresponding to an instability state of the modulator, and a circuit for monitoring the output bit stream of the modulator. The circuit further includes a circuit for sending resetting signals to at least one of the integrator stages when the instability sequence is recognized in the output bit stream.
In accordance with another aspect of the invention, a method of re-establishing the stability of a sigma-delta modulator having a plurality of integrator stages in cascade along with a quantizer is provided. The method includes defining a bit sequence corresponding to an instability state of the modulator, monitoring the output bit-stream of the modulator to check whether it comprises the instability sequence, and, when the instability sequence is detected, resetting the last integrator stage and, progressively, resetting one or more preceding integrator stages until the instability sequence is no longer detected.